Hour glass puzzle

#1

i am not understand . what is happening . How to measure 15 minutes

#2

hello @vishal_sangal_123
you first turn on both glasses, so when one glass goes off, time remaining in other glass is 4 minutes(11-7), now when this 4 minutes are over, again turn on the glass with 11 minutes, so now you measured 15 minutes(4+11), total time taken to measure 15 minutes is 22 minutes.(as you waste 7 mins at starting to measure 4 minutes)